A field effect transistor (hereinafter referred to as “FET” in some cases) has a gate electrode, a source electrode, and a drain electrode. In addition, the field effect transistor is an active device in which current flowing in the channel layer, that is, current flowing between the source and the drain electrodes is controlled by applying a voltage to the gate electrode. In particular, an FET using a thin film formed on an insulating substrate of a ceramic, a glass, or a plastic as a channel layer is called a thin film transistor (hereinafter referred to as “TFT” in some cases).
The TFT can be advantageously easily formed on a relatively large substrate by using techniques to form thin films, and as a result, the TFT has been widely used as a switching device and driver device of a flat panel display such as a liquid crystal display. That is, in an active matrix liquid crystal display (ALCD), ON/OFF operation of each image pixel is performed using a corresponding TFT formed on a glass substrate. In addition, in high-performance organic LED display (OLED), effective control of the pixel current is expected to be performed by a TFT. Furthermore, a liquid crystal display device having higher performance has been realized in which driver circuits composed of TFTs are formed on a substrate in a peripheral region of an image region.
The most widely used TFT in these days uses a polycrystalline silicon film or an amorphous silicon film as a channel layer material. The TFT described above is called a metal-insulator-semiconductor field effect transistor (MIS-FET). For a pixel driver, an amorphous silicon TFT is used, and for peripheral circuits of driver and controller, a high-performance polycrystalline silicon TFT has been practically used.
However, in TFTs including an amorphous silicon TFT and a polycrystalline silicon TFT which have been developed heretofore, high temperature processes are required for device formation, and hence it has been difficult to form the TFT on a substrate such as a plastic plate or film.
On the other hand, in recent years, development to realize flexible display has been actively performed by forming TFTs on a polymer plate or film as drive circuits of LCD or OLED. As a material which can be formed on a plastic film or the like, an organic semiconductor film has drawn attention which can be formed at a low temperature.
For example, as an organic semiconductor film, research and development of pentacene and the like has been implemented. The organic semiconductors mentioned above all have aromatic rings, and when they are crystallized, a high carrier mobility can be obtained in the lamination direction of aromatic rings. For example, it has been reported that when pentacene is used for an active layer, the carrier mobility is approximately 0.5 cm (Vs)−1 and is approximately equivalent to that of an amorphous Si-MOSFET.
However, the organic semiconductors such as pentacene disadvantageously have inferior heat stability (<150° C.) and toxicity (oncogenic properties), and hence practical devices have not been realized as of today.
In addition, in recent years, as a material which can be applied to a channel layer of the TFT, oxide materials have begun to draw attention.
For example, development of TFT has been actively pursued which uses a transparent and conductive polycrystalline oxide thin film primarily composed of ZnO as a channel layer. The above thin film can be formed at a relatively low temperature and hence can be formed on a substrate such as a plastic plate or film. However, since a compound primarily composed of ZnO cannot be formed into a stable amorphous phase at room temperature but is formed into a polycrystalline phase, the electron mobility cannot be increased due to scattering at interfaces of polycrystalline grains. In addition, since the shape of polycrystalline grains and connection therebetween are significantly changed by film-forming methods, the properties of TFTs are scattered.
Recently, a thin film transistor has been reported in which an amorphous In—Ga—Zn—O-based oxide is used (K. Nomura et al, Nature Vol. 432, p. 488 to 492, November 2004). This transistor can be formed on a plastic or a glass substrate at room temperature. In addition, at a field-effect mobility of approximately 6 to 9, normally-off type transistor properties can be obtained. Furthermore, this transistor is advantageously transparent to visible light.
When the inventors of the present invention investigated a thin film transistor using an amorphous In—Ga—Zn—O-based oxide, although depending on the composition and manufacturing conditions, the hysteresis of transistor properties (Id−Vg properties) of TFT was generated in some cases.
When the above TFT is used, for example, for a pixel circuit of a display device, the generation of the hysteresis as described above causes variation in operation of organic LED, liquid crystal, or the like which is to be driven, and as a result, the image quality of the display device is degraded.